Guanine nucleotide-binding proteins (GTP-binding proteins, or G proteins) participate in a wide range of regulatory functions including metabolism, growth, differentiation, signal transduction, cytoskeletal organization, and intracellular vesicle transport and secretion. These proteins control a diverse sets of regulatory pathways in response to hormones, growth factors, neuromodulators, or other signaling molecules. When these molecules bind to transmembrane receptors, signals are propagated to effector molecules by intracellular signal transducing proteins. Many of these signal transducing proteins are GTP-binding proteins.
Low molecular weight (LMW) GTP-binding proteins are small proteins which consist of single polypeptides of 21-30 kDa. These proteins regulate cell growth, cell cycle control, protein secretion, and intracellular vesicle interaction. In particular, the LMW GTP-binding proteins activate cellular proteins by transducing mitogenic signals involved in various cell functions in response to extracellular signals from receptors (Tavitian, A. (1995) C. R. Soc. Biol. Fil. 189:7-12). During this process, the hydrolysis of GTP acts as an energy source as well as an on-off switch for the GTPase activity of the LMW GTP-binding proteins.
The LMW GTP-binding proteins are classified into five subfamilies: Ras, Rho, Ran, Rab, and ADP-ribosylation factor. Despite their sequence variations, all five subfamilies share common conserved structural features. Four sequence regions, termed motifs I-IV, are conserved in the LMW GTP-binding proteins. Motif I is the most variable and has the signature, GXXXXGK. The lysine residue is essential in interacting with the .beta.- and .gamma.-phosphates of GTP. Motif II, III, and IV are highly conserved, with DTAGQE, NKXD, and EXSAX as their respective signatures. These motifs regulate the binding of .gamma.-phosphate, GTP, and the guanine base of GTP, respectively. Most of the membrane-bound LMW GTP-binding proteins generally require a carboxy terminal isoprenyl group for membrane association and biological activity. The isoprenyl group is added posttranslationally by a mechanism which recognizes a terminal cysteine residue alone or a terminal cysteine-aliphatic amino acid-aliphatic amino acid-any amino acid (CAAX) motif. Additional membrane-binding energy is often provided by either internal palmitoylation or a carboxy terminal cluster of basic amino acids. The LMW GTP-binding proteins also have a variable effector region, located between motifs I and II, which is characterized as the interaction site for guanine nucleotide exchange factors (GEFs) or GTPase-activating proteins (GAPs). GEFs induce the release of GDP from the active form of the G protein, whereas GAPs interact with the inactive form by stimulating the GTPase activity of the G protein.
The Ras subfamily proteins already indicated supra are essential in transducing signals from receptor tyrosine kinases (RTKs) to a series of serine/threonine kinases which control cell growth and differentiation. Mutant Ras proteins, which bind but cannot hydrolyze GTP, are permanently activated and cause continuous cell proliferation or cancer. TC21, a Ras-like protein, is found to be highly expressed in a human teratocarcinoma cell line (Drivas, G. T. et al. (1990) Mol. Cell. Biol. 10: 1793-1798). Rin and Rit are characterized as membrane-binding, Ras-like proteins without the lipid-binding CAAX motif and carboxy terminal cysteine (Lee, C.-H. J. et al. (1996) J. Neurosci. 16: 6784-6794). Further, Rin is shown to localize in neurons and have calcium-dependant calmodulin-binding activity.
The discovery of a new human Ras-like protein and the polynucleotides which encode it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of inflammation and disorders associated with cell proliferation and apoptosis.